\documentclass[10pt]{article} % required
\pagestyle{empty} % required
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{color}
\usepackage[T1]{fontenc}

\begin{document}\definecolor{fgC}{rgb}{0,0,0}\color{fgC}\[\begin{matrix}
 K_1 = U^n \\ 
 K_2 = 0 \\
\left.\begin{matrix}
 K_2 = A_s K_2 + \Delta t R(t^n + C_s \Delta t, K_1) \\ 
 K_1 = K_1 + B_s K_2
\end{matrix}\right\} s=1,2,...N_s\\
U^{n+1} = K_1
\end{matrix}
\]\end{document}